1. Field of the Invention
The present invention relates to an ion sensitive field effect transistor, and more particularly to the formation of a thin metal film by thermal evaporation or RF reactive sputtering as a method of fabricating a light shielding layer for an ion sensitive field effect transistor.
2. Description of the Related Art
A conventional ion transistor, for example a glass electrode, has good stability and can choose ions accurately. However, it is large and expensive, and its reaction time is lengthy. Therefore, the advanced technology of the silicon semiconductor integration circuit for ion sensitive field effect transistors will eventually replace the glass electrode.
In 1970, Piet Bergveld removed the metal film of the general MOS transistor above the gate and put it into an ion solution. As the solution changed in the concentration of H.sup.+, the oxide layer above the gate acted as an ion sensitive film and caused the voltage of the interface between the film and the solution to change. Further the amount of current through the channel changed. By the way of measuring the changing amount of current, it was possible to measure the concentration of H.sup.+ and the other ions. Thus, Piet Bergveld called his research the ion sensitive field effect transistor.
In the 1970's, the process and application of ion sensitive field effect transistors were in a research stage, while in the 1980's, the study of ion sensitive field effect transistors included not only basic theory, but also advanced into practical application. In fact, over 30 kinds of ion sensitive field effect transistors used to measure the ions and chemical materials were developed. The ion sensitive field effect transistors decreased in size and improved in design and function. Further, they had characteristics that the conventional ion transistor didn't have such as:
1. Small size, allowing the measurement of the ion concentration of a smaller amount of solution;
2. High input resistance and low output resistance;
3. Quick response; and
4. Process compatibility with MOS field effect transistors.
However, there are two points that need to be overcome for the practical application of ion sensitive field effect transistors. One is the effect of temperature, and the other is the influence of surrounding light while measuring. Regarding the first point, the effect of temperature can be lowered by means of complementary circuits. U.S. Pat. No. 5,319,226 (Jun. 7, 1994) discloses an ion sensitive field effect transistor in which the formation of a Ta.sub.2 O.sub.5 thin film above the gate of ion sensitive field effect transistor results in the construction Ta.sub.2 O.sub.5 /Si.sub.3 N.sub.4 /SiO.sub.2. With reference to FIG. 1, the construction of the ion sensitive field effect transistor and its operation can be specifically realized.
As shown in FIG. 1, the construction and operation of this kind of transistor comprise: forming a transistor having a drain D and a source S by ion implantation in a semiconductor substrate 10; forming a Si.sub.3 N.sub.4 /SiO.sub.2 isolating layer 12 above the drain and the source; utilizing RF reactive sputtering to form a Ta.sub.2 O.sub.5 thin film 14 above the gate isolating portion. After this, the Ta.sub.2 O.sub.5 thin film 14 contacts the solution 16 directly so as to react with the hydrogen and changes its concentration. Accordingly, the voltage on the surface of the Ta.sub.2 O.sub.5 thin film 14 changes, and the amount of current through the drain changes. Therefore, the pH measurement can be obtained by detecting the drain voltage Vd, where the drain voltage Vd represents an opposed voltage to a reference voltage Vr, while the reference voltage Vr is defined by a reference electrode 18 that contacts the solution 16.
Regarding the influence of light, the materials used in sensing membrane, for example Si.sub.3 N.sub.4, Ta.sub.2 O.sub.5, Al.sub.2 O.sub.3, and SnO.sub.2, are materials whose energy bands are wide (over 3.5 eV). Accordingly, light will go through these membranes, form electron-hole pairs, and cause errors in the measurement. Therefore, when using this kind of transistor, it is possible to be sure that the measurement is accurate only if the light intensity of the measuring environment is constant.